The objective is to understand, on multiple levels of biological organization, the mechanisms of pattern regulation in regenerating amphibian limbs. The specific aims are: (1) To characterize the effects of retinoids on morphological pattern in the three cardinal axes of regenerating and embryonic amphibian limbs. This will be accomplished by removing pattern from each axis followed by injection of retinoids over a wide dose range and at different developmental stages, then examining the types and degrees of pattern abnormalities in the resultant regenerates. (2) To determine the histological and ultrastructural correlates of retinoid-induced alterations in pattern of regenerating and embryonic limbs, and determine the relationships between retinoid-induced increase in pattern and blastema volume, cell number and labeling index. This will be done by conventional histological techniques, transmission and scanning electron microscopy, and computerized three-dimensional reconstruction of patterns of labeling and cell density. (3) To use retinoids to test the hypothesis that there are position-related differences in blastema cell recognition/affinity that are linked to positional memory. To do this, an in vivo assay system will be used to determine whether retinoids coordinately modify recognition/affinity and positional memory. (4) To determine, by high-performance size-exclusion chromatography, whether regenerating limbs contain retinoid binding proteins, which may mediate the intracellular action of retinoids in other systems. These aims are of significance to human health because of the insight they will provide into the problem of regenerating vital human tissues and organs, understanding the origins of congenital defects, and understanding the mechanisms whereby teratogens and drugs affect developing tissues and organs.